1. Field of the Invention
The present invention relates to a rewritable phase-change optical information recording medium having a large recording capacity and capable of rewriting information recorded therein.
2. Discussion of Background
As optical information recording media which are capable of recording information therein, and from which recorded information can be reproduced or erased by the application of semi-conductor laser beams thereto, there are conventionally known (1) a magneto-optical recording medium in which information can be recorded, and recorded information can be erased by reversing a magnetization direction of a recording layer thereof, utilizing heat, and (2) a phase-change optical information recording medium in which information can be recorded, and recorded information can be erased, utilizing phase changes between a crystalline phase and an amorphous phase of a recording layer thereof.
The latter phase-change optical information recording medium is capable of performing single beam overwriting, and is advantageous over other optical recording media in the compatibility with CD-ROM and CD-R media, so that the standardization of the phase-change optical information recording media as rewritable media, namely as CD-RW, has now been established, and the phase-change optical information media have now been commercialized.
In the meantime, large capacity storage by the phase-change optical information recording media has been studied, and DVD-ROM media have been placed on the market. In accordance with the apparatus of the DVD-ROM on the market, media called DVD-RAM are now being developed as rewritable DVD media. A DVD-RAM with a capacity of 2.6 GB will be shortly commercialized, but there is now a demand for a DVD-RAM with a capacity larger than that of ROM media.
As the materials for use in the recording layer of the phase-change optical information recording medium, chalcogen-based alloys, such as Ge--Sb--Te, In--Sb--Te, Ge--Se--Te, Ge--Te--Bi, Sb--Se--Te, and In--Te--Au have been investigated. Of these chalcogen-based alloys, Ge--Sb--Te has now reached a level for use in practice. However, even with this Ge--Sb--Te, further improvements are desired on the recording sensitivity, erasing sensitivity, and erasing ratio at overwriting.
Japanese Laid-Open Patent Applications 4-78031 and 9-263055 disclose phase-change information recording media using as recording materials in the recording layers thereof Ag--In--Sb--Te based alloys by use of which the erasing ratio of the phase-change information recording media at the overwriting is improved.
However, the repeated use overwriting characteristics of a phase-change optical information recording medium using the above-mentioned Ag--In--Sb--Te based recording material cannot be improved by using the Ag--In--Sb--Te based recording materials only, but by using upper and lower protective layers, and a heat dissipation layer which are overlaid.
As the materials for such protective layers, ZnS.SiO.sub.2 as disclosed in Japanese Patent Publication 7-114031, metallic oxides, metallic sulfides and metallic nitrides, and mixtures thereof are conventionally proposed. However, the improvement of the repeated use overwriting characteristics of the phase-change optical information recording medium is still insufficient for use in practice.
As mentioned above, the phase-change optical information recording media have now been commercialized as CD-RW. The CD-RW is used as an external memory device for use with personal computers. Recently DVD-ROM players are placed on the market, so that there is a great demand for large capacity, rewritable DVD media.
The DVD-RAM which has now been commercialized as rewritable DVD medium has a capacity of 2.6 GB, and recording and reproduction is conducted at a linear speed of about 6 m/sec, while in DVD-ROM, the linear speed for recording and reproduction is about 3.5 m/sec, and the capacity of DVD-ROM is 4.7 GB. It is required that rewritable DVD media have a capacity greater than that of ROM, be capable of peforming recording and reproduction at a speed of two times greater than that of ROM, and be compatible with ROM. In order to meet such requirements, it is strictly required that the DVD media have a high line density, a low jitter, and a large overwriting repetition number. In particular, the improvement of the overwriting repetition number is one of most significant targets to be cleared in the phase-change optical information recording media in order to enhance the performance reliability of the media. The Ag--In--Sb--Te based phase-change recording materials are suitable for high density recording, so that how to improve the overwriting repetition characteristics thereof is still a significant target to be cleared.